The present invention relates to voltmeters generally and to voltmeters for use in electrical power transmission line servicing and maintenance in particular.
Electricity transmitted through power lines destined for commercial, industrial and residential use can involve hundreds of thousands of volts and high currents. Inevitably, there is an element of danger in measuring the voltage on a transmission line because of the need to make contact with the line. Indeed, even the proximity to a high voltage line may be sufficient to cause a spark to jump through the air to the nearest object. Nonetheless, in installing, servicing and repairing power lines, there are various occasions when contact is made, such as when the voltage carried by a line must be measured.
The circumstances and equipment used for measurements of the voltage of transmission lines varies considerably. For example, the absolute voltage carried by a line may be measured by a xe2x80x9chigh line resistive voltmeter.xe2x80x9d As another example, in servicing or repairing voltage regulators, an xe2x80x9coff neutral detectorxe2x80x9d is used to determine if the regulator is passing current or has been effectively isolated from the power source. In still another application, a xe2x80x9cphasing voltmeterxe2x80x9d is customarily used in connecting individual lines of the multi-phase transmission power lines. The phasing voltmeter helps to prevent two lines that are not in phase from being connected inadvertently. Phasing voltmeters are not very accurate and generally do not need to be in order to indicate which lines are in phase and which are not. For accuracy, high line resistive voltmeters are used.
However, if a phasing voltmeter were accurate, it may have additional uses, such as replacing the high line resistive voltmeter or the off neutral detector. In order to be accurate, an alternating current phasing voltmeter should be capable of making four very distinctly different voltage measurements: phase to phase (FIG. 2A), phase to ground (FIG. 2B), ground to phase (FIG. 2C), and zero reference test (FIG. 2D). This last measurement should indicate very nearly zero volts when measuring the voltage difference between two conductors of the same phase and voltage or between two points on the same conductor.
Presently, high voltage phasing voltmeters use two high voltage resistors in series with each other and a meter and a cable. The resistors are housed in two insulated holders that are connected to the series cable and the series meter. The holders will have metal hooks or other fittings on their ends for good electrical contact with transmission lines. Often the meter is mounted to one of the two insulated holders (see FIG. 1) and oriented so that the electric utility worker can read the voltage displayed on the meter. xe2x80x9cHot sticksxe2x80x9d may be used to hold and elevate the entire assembly. The meter may be designed to measure either voltage or current, but its display indicates voltage. However, the indicated voltage is not always the true voltage difference for the four types of measurements listed above.
High voltage measurements are plagued with inaccuracies stemming from stray capacitive charging currents. At high voltages, these stray currents emanate from the surface of every component of the measuring device including the cable. The capacitive current is related to the capacitive reactance, Xc, which can range from several thousand ohms on up, depending on the position of the meter and cable with respect to the ground. Under extreme conditions, such as when the series cable is lying directly on the ground between two pad-mounted transformers, the value of the capacitive reactance can be very low. The resulting capacitive current can then equal or exceed the measured current. Moreover, the voltage measured by the meter varies depending on the location of the meter and cable.
To demonstrate the theoretical limits of the inaccuracy of the prior art phasing voltmeter, assume that the capacitive current goes to a maximum (capacitive reactance goes to zero). This situation would electrically ground the series cable. The inaccuracies for the four basic measurements would be 15% too high for phase to phase, 100% too high for phase to ground, zero for ground to phase, and 200% of line to ground voltage for the zero reference test.
If the inaccuracies in phasing voltmeters attributable to capacitive currents could be eliminated, phasing voltmeters could be used to make measurements currently made by high line resistive voltmeters and off neutral detectors. This would eliminate the need for these additional voltmeters and provide measurements in which electrical utility employees can have greater confidence.
The problem of the inaccuracies introduced by the capacitive currents is known, and there have been attempts to address this problem. One solution is to keep the current actually measured by the phasing voltmeter relatively large compared to the capacitive current so the error introduced by the latter is relatively small. However, larger currents carry with them heat that can affect the resistance of the high voltage dropping resistors, which introduces another source of error if the phasing voltmeter is kept in contact with the line too long. Bevins in U.S. Pat. No. 3,392,334 teaches that the cable must be kept as short as possible and that use of two conductors in the interconnect cable can nullify the effects of capacitive current. However, neither of these steps corrects the error. For example, a phase to ground reading with a phasing meter having dual conductors in the cable will be the same as the ground to phase reading, but both will understate the actual voltage. Keeping the cable short certainly helps with accuracy but makes the phasing voltmeter less useful than one with a longer cable.
Thus there remains a need for a phasing voltmeter that is accurate regardless of the capacitive current.
According to its major aspects and briefly recited, the present invention is a phasing voltmeter where the capacitive currents are combined with the primary voltage measurement of the electrical transmission lines in such a way that the capacitive current has no net affect on the voltage measured regardless of the magnitude of the capacitive current. Thus, the meter and cable can be moved about without affecting the primary voltage measurement, and the cable can be of any length.
The present phasing voltmeter includes a pair of high impedance resistors in series with a cable and a high impedance alternating current (AC) voltmeter, essentially similar to the prior art phasing voltmeters, but also having a low impedance electrical circuit in parallel with the meter and that is tied at a single point of contact to electrical shielding provided for the resistors, cable and AC voltmeter. This shielding picks up the capacitive currents in the vicinity of the phasing voltmeter.
By this arrangement, the present phasing voltmeter establishes three voltage divider networks. The first voltage divider network divides the source voltage by an exact amount and provides a precise voltage to the AC voltmeter. The second voltage divider network divides the voltage between the first high impedance resistor and the single point shield attachment. The third voltage divider network divides the voltage between the second high impedance resistor and the A single point shield attachment. The vector sum of the three voltages thus established across the AC voltmeter input from these three voltage divider networks is exactly proportional to the source voltage. Thus, the second and third voltage divider networks allow the capacitive current to be coupled to the measured voltage in such a way that it has no net effect on the measured voltage.
A number of different embodiments of this electrical circuit are described herein, but a preferred one includes three resistors in series. The first is an adjustable gain resistor. The second is a null resistor tied to the shielding. The third is a balance resistor. Before the first use of the present phasing voltmeter, the gain and null resistors are set so that the particular system is balanced. Thereafter, no further adjustments are needed and the voltmeter will read true.
By adding switches and additional gain, null and balance resistors in accordance with the present invention and replicating, the electrical circuit can be established to provide range selection so that the displayed voltage will fall into one of a plurality of ranges selected by the user.
A feature of the present invention is the electrical circuit in parallel with the AC voltmeter. This simple circuit allows the capacitive current to be combined with the measurement of the voltage in such a way that there is no net effect regardless of the magnitude or changes in the capacitive current. This result has a number of advantages.
First, it eliminates what is potentially a large source of error in the measurement of voltages using phasing voltmeters, thus making much more accurate measurements possible. An accurate phasing voltmeter has other uses, as mentioned above. Moreover, an accurate phasing voltmeter helps electrical utility employees engaged in the servicing and repair of power lines to avoid injuries.
Second, it allows the measurement to be done anywhere and with any length of cable. A longer cable increases capacitive currents but as long as there is no net affect on the voltage measurement, it does not matter how long the cable is.
Third, the present invention allows the voltmeter to make its measurements on much smaller currents because the capacitive current is not a factor. Prior art meters reduced the impact of capacitive current by keeping the current running through the high voltage resistors and the meter relatively higher. Because lower currents can be used with the present device, resistors are kept cooler and can be used longer without heating up and causing the electrical resistence coefficient to drift.
These and other features and their advantages will be apparent to those skilled in the art of transmission line voltage measurement from a careful reading of the Detailed Description of Preferred Embodiments accompanied by the following drawings.